You best bet it to do a complete format and reinstall in order to confirm all your data is erased. Without the CDs you will basically have to backup all your data to an external drive, delete anything and everything personal. All documents, all the application stores in the library and empty your trash. Even after you do all that, there is still the possibility of someone recovering your data. You really need some install CDs. Or just take out the hard drive and sell it without a hard drive or OS.